Broad objectives of our research are to understand the cellular and molecular mechanisms which result in transmission of the human blood fluke Schistosoma mansoni in certain individual host snails (Biomphalaria glabrata), and the failure of transmission in others. This requires knowledge of the events which culminate in compatibility and those that result in incompatibility. Specifically we seek to identify parasite-borne characters which modulate infectivity and host-borne characters which determine susceptibility or resistance. In order to describe molecular events involved in parasite recognition by the intermediate host snail, the complex sporocyst- hemocyte system which occurs in vitro will be simplified for experimental analysis. Sporocyst surface components will be placed on activated beads. Hemocyte recognition of these sporocyst- labelled beads will be assessed by association of the two in vitro. Association indices will be obtained for hemocytes from snails which are susceptible to the schistosome and those from snails which are resistant. Hemocytes from naive and from parasite- exposed snails will be compared. The influence of snail plasma will be evaluated in experiments where the sporocyst-labelled beads are exposed to plasma before they are offered to hemocytes. Plasma will be derived from both susceptible and resistant snail strains, and from naive parasite-exposed individuals. Recognition factors on hemocyte surfaces and those from the plasma will be studied for their influence on the outcome of hemocyte- sporocyst encounters using a well tried cytotoxicity assay system.